1. Field of the Invention
The present invention relates to a zoom lens unit, an imaging device, and a portable information terminal device.
2. Description of the Related Art
In recent years, digital cameras have become popular. The demand of users for digital cameras varies over a wide range. In particular, a user always requires high image quality, and a reduction of size of the camera, and both the high performance and compactness are required for a zoom lens unit which is used as a photographing lens.
From the viewpoint of compactness of a zoom lens unit, it is necessary to shorten a total length of the zoom lens unit (a distance from a lens surface nearest to an object side to an image surface) when using it. In addition, it is important to shorten a thickness of each lens group to reduce a total length of the zoom lens unit when the zoom lens unit is collapsed.
From the viewpoint of high performance of a zoom lens unit, at least a resolution corresponding to an imaging element having 10 to 15 million pixels over all a zoom area is necessary for the zoom lens unit.
In addition, many users require a photographing lens with wide-angle capability, and it is desirable that a half-field angle of a zoom lens unit at a wide-angle end be 38 degrees or more. The half-field angle of 38 degrees corresponds to a focal length of 28 mm in terms of a 35-mm silver salt camera (so-called Leica version).
In addition, a large magnification ratio is preferably desired. A zoom lens unit with a focus length of about 28-200 mm (about 7.1 times) in terms of a 35-mm silver salt camera is considered to be capable of responding to almost all general photographing conditions.
Many kinds of zoom lens units for a digital camera are possible. As a kind of zoom lens unit which is suitable for a high magnification ratio, the following zoom lens unit is known, which includes, in order from an object side to an image side, a first lens group having a positive focal length; a second lens group having a negative focal length; a third lens group having a positive focal length; and a fourth lens group having a positive focal length, and when changing magnification from a wide-angle end to a telephoto end, an interval between the first lens group and the second lens group increases, an interval between the second lens group and the third lens group decreases, and an interval between the third lens group and the fourth lens group varies.
As this kind of zoom lens unit, when changing magnification, the first lens group can be fixed or reciprocate with a convex trajectory to the image side. If it is attempted to secure a great displacement of the second lens group which bears a major changing magnification function, an aperture stop disposed in the neighborhood of the third lens group moves away from the first lens group even at the wide-angle end, and a size of the first lens group tends to increase when attempting to obtain a wide angle and high magnification ratio.
With the above-mentioned kind of zoom lens unit, to obtain a compact zoom lens unit with a wide angle and a high magnification ratio, it is preferable that the first lens group be moved such that the first lens group locates nearer to the object side at the telephoto end than at the wide-angle end, to change the magnification. Thus, by shortening the total length of the zoom lens unit at the wide-angle end comparing with that at the telephoto end, it is possible to reduce an increase in size of the first lens group while obtaining a sufficient wide angle.
Chromatic aberration is easy to generate associating with a high magnification ratio and long focus length, and it is well-known that utilization of a lens with anomalous dispersibility is effective for correction of the chromatic aberration.
JP H08-248317 A, JP 3391342 B, JP 2004-333768 A and JP 2008-026837 A propose a zoom lens unit that employs a lens with anomalous dispersibility in the above-mentioned kind of zoom lens unit.
In a zoom lens unit disclosed in JP H08-248317 A, a half-field angle at a wide-angle end is 25 degrees. In an example of a zoom lens unit constituting of four lens groups having positive/negative/positive/positive refracting power, disclosed in JP 3391342 B, a half-field angle at a wide-angle end is about 29 to 32 degrees. These zoom lens units disclosed in JP H08-248317 A and JP 3391342 B are not ones that sufficiently satisfy the request nowadays in the point of a wide angle.
A zoom lens unit disclosed in JP 2004-333768 A has a wide angle, in which a half-field angle at a wide-angle end is about 37 degrees, but as a whole the zoom lens unit constituting many lenses, as many as 14 lenses, leads to problems of shortening of a total length of the zoom lens unit when collapsed and decreasing cost.
In a zoom lens unit disclosed in JP 2008-026837 A, a wide angle and high magnification ratio have been achieved with a relatively simple structure, however a total length of the zoom lens unit at a telephoto end is longish, therefore there is still room for improvement in reduction of size of the zoom lens unit.